3. Hydrogeological interpretation
3.1 Geological setting
A summary geology map for the Waitaki Catchment (Figure 2A and 2B) shows that the predominant geology of the aquifer basins in the catchment consists of Pleistocene and postglacial gravel deposits (Plate 2 - Figure 1) surrounded and underlain by Tertiary, Cretaceous and basement rocks.
There are, however, subtle differences in the composition of gravels and sediments within the Waitaki Catchment. Moraine areas located in the Tekapo and Twizel Basins contain poorly sorted sediments and glacial till. This implies that groundwater hydraulics will be low and bore yields in these areas is likely to be poor. There is also a difficulty in correlating piezometric surfaces within moraine areas compared to that of alluvial outwash areas, because of the alternating coarse and fine deposits, which can produce to localised perched water tables. The presence of moraine areas within basins also provides some topographical variation to that of the 'flatter' alluvial terraces. In the recent alluvial gravels of the lower Waitaki alluvium, bore yields are likely to be much greater given the higher range in permeability for these gravels
Major faulting occurs in the Waitaki Catchment and generally defines specific sub-catchments or basins. In the lower catchment, the Waitaki Fault delineates the sedimentary rocks to the south from the Lower Waitaki Alluvium. The Waitaki Fault tends to diminish near Black Point, where greywacke and schist basement outcrops. Occurrences of recent alluvium in the Lower Waitaki Catchment are variable from thin veneers surrounding local stream outwash areas to a deep alluvial trench (the Kurow Trench) associated with a small graben in vicinity of the Penticotico Stream area.
Waitaki Catchment geology delineates groundwater basins, which are mapped in Figure 1. However, exact delineations of some zones on the basis of geology alone is not possible, due to watershed or groundwater divides. The areas where there are 'soft' groundwater divides or uncertainty in geological extent within zones, have been shown using dashed lines for individual basins (Figure 1).
A schematic hydrogeological cross-section oriented south to north through the lower Waitaki and Glenavy basins is reproduced from the URS Project Aqua Report in Figure 3. Note that hydrogeological cross-sections could not confidently be provided for other areas of the Waitaki Catchment due to limited bore lithological and groundwater information.
Figure 2A & 2B. Summary geology map for the Waitaki Catchment
(A3 full version of images is available in paper format only)
Figure 3. Schematic hydrogeological cross-section through the lower Waitaki alluvium (sourced from URS, 2003b)
Plate 2. Pukaki Basin terrace adjacent to the Pukaki River
3.2 Aquifer zones
Various groundwater basins with key surface water features have been identified in this report for the Waitaki River Catchment (Figure 1). The groundwater zones have been drawn on the basis of topographical and hydrological boundaries, geological control, and known and assumed groundwater flow direction in unconfined aquifers as discussed in Section 3.4. The 13 groundwater basins and their respective surface areas and surface water features are summarised in Table 1.
Table 1. Summary of groundwater basins
View summary of groundwater basins (large table)
In the upper Waitaki Catchment, the Tekapo Basin is separated from the Twizel Basin on the basis of groundwater flow and relief features. However, the boundary of these zones is unclear in the upper Pukaki area and the extent of the Tekapo Basin in the upper Irishman Creek area is unknown, as little information is available on surface and groundwater hydrology in these locations (Section 4).
A groundwater divide separates the Twizel and Omarama basins in the vicinity of Clear Burn, between the Ahuriri River and Lake Ruataniwha. However, the exact position of the divide is not known in any detail, and is shown as a dashed line on Figure 1.
The upper Omarama Basin is separated from the Omarama Basin to represent the drainage features of the Quail Burn and Hen Burn streams. These two streams drain the basin through the 'Clay Cliffs' to the Ahuriri River. Groundwater divides exist to the north and south of the upper Omarama Basin, separating the Twizel and Omarama basins respectively.
The areas of the north and south bank of the Waitaki River below Kurow are also separated from the lower Waitaki Plain below Black Point. This has been delineated on the basis of piezometric contouring. It should be recognised that the boundaries may change over time, reflecting changes in state of the water table, irrigation season and Waitaki River flow.
Other areas such as the Benmore Terrace, Aviemore and Waitaki Flats have little or no information available on groundwater resources. The basins have been delineated (Figure 1) as groundwater yields may be available from these areas. The Maerewhenua Basin and Papakaio Formation are located to the south of the lower Waitaki Valley. These groundwater areas are separated by depth, with the Papakaio Formation underlying much of the Maerewhenua River alluvium.
